The general consensus is to only use a terminal server as a terminal server - to run any other major service is overhead and will negatively impact overall performance.
You could do it - if the only reason for doing so is so that you can log in remotely and get a GUI screen.Seems a bit of a waste of the additional expense, though.
For each users' terminal session you will need an additional 32Mb or so of RAM in the server - depending on what is run in the session.
To answer your second question, NT4 TSE has a rewritten kernel - it is not just NT4 with an added service.
Hope this helps